The chief characteristics required of a honeycomb carrier to support a catalyst to be used for an apparatus for cleaning an automobile exhaust gas which is particularly widely used at present among various apparatuses for cleaning an exhaust combustion gas, are sol-called heat resistance and thermal shock resistance. A high heat resistance is required since the honeycomb carrier will be exposed to a high temperature of 850° C. or higher by sudden heat generation due to a catalytic oxidation reaction of unburned hydrocarbons or carbon monoxide in the exhaust gas. Further, the thermal shock resistance is a quality to be resistant to cracks or breakage by a thermal stress caused in the honeycomb due to a temperature increase by such sudden heat generation. With respect to the thermal shock resistance, the smaller the thermal expansion coefficient, the greater the endurance temperature difference.
So as to meet such requirements as heat resistance and thermal shock resistance, various ceramics have been proposed as a material for a honeycomb carrier, but a cordierite material has been chiefly used. The primary reason why a cordierite material is used is that cordierite has a so high thermal resistance as 1,400° C., and it has an extremely small thermal expansion coefficient and high thermal shock resistance among ceramics as well.
However, although a cordierite material as a material for a honeycomb carrier has rather excellent quality with respect to heat resistance and thermal shock resistance, it is highly disadvantageous when used as a catalyst carrier for cleaning an exhaust gas containing a nitrogen oxide (NOx), the removal of which is urgently required from an environmental viewpoint. That is, usually a catalyst containing an alkali metal or alkaline earth metal component is used as a catalyst to remove NOx in the exhaust gas. In such a case, a part of the alkali metal or alkaline earth metal is infiltrated into cordierite as a carrier and reacts with cordierite at a high temperature, and such leads to a deterioration of cordierite and loss of the catalyst as well, and thus causes a decrease of removal of NOx in the exhaust gas. In order to prevent such a phenomenon, a method of covering the surface of the catalyst with silica (SiO2), and the like, have been proposed, but an extra step will be required, and an increase in the cost will be inevitable.
On the other hand, in a system wherein a fuel is directly jetted into an engine or in a system wherein a fuel is diluted and burned, which is becoming the main stream of a burning system of an automobile in recent years from a viewpoint of improvement in mileage and from an environmental viewpoint, removal of NOx in the exhaust gas is a particularly important concern as compared with removal of hydrocarbons and carbon monoxide. Accordingly, as a material for a honeycomb carrier to support a catalyst to clean an exhaust gas, a material which replaces cordierite has been strongly desired.
As materials other than cordierite, WO01/037971 discloses ceramics such as silicon carbide, silicon nitride, mullite, aluminum titanate and lithium aluminum silicate. However, they are all insufficient as a material for the honeycomb carrier. That is, silicon nitride, mullite, etc. have a high thermal expansion coefficient and are poor in thermal shock resistance. Further, silicon nitride, lithium aluminum silicate, etc. are insufficient in view of heat resistance.
Aluminum titanate has excellent stability even at a high temperature exceeding 1,700° C., an extremely small thermal expansion coefficient and excellent thermal shock resistance. However, it has such a drawback as small mechanical strength since the anisotropy of its crystal structure is significant, whereby slip is likely to occur at the crystalline interface by a thermal stress. Resultingly, a honeycomb having a small wall thickness and a high cell density is hardly produced with it, and its use as a carrier for an exhaust gas-cleaning catalyst to which a load of mechanical vibration will be applied at a high temperature, tends to be difficult. Further, such aluminum titanate, etc usually have decomposition points within a temperature range of from 800 to 1,280° C., and they can not be used continuously for a long time in a region including such a temperature range.